Last time I was at Tempe Camera (the local pro camera shop), they had a mint-condition 50 f/1.0L on consignment. Just for giggles, I mounted it to my 5DIII. Wide open, it's a coke bottle.

I don't remember what they were asking for it, but it was pretty reasonable -- in line with the going rate. I keep thinking that, if they still have it the next time I'm there, I'll offer them an insultingly low amount for it...and then I remember that it's probably not even worth as much as the 50 f/1.2L lens to me. Realistically, the lens has no use beyond the giggle factor of seeing the meter read 1.0.